Raising San Vicente Dam: Why and How

The San Diego County Water Authority is preparing to raise the 220-foot-high San Vicente Dam by as much as 117 feet. This dam raise, which will be accomplished using roller-compacted concrete, is intended to increase water supply reliability for southern California. This will be the largest raise of a concrete dam in the U.S. and the largest raise in the world using RCC.

San Diego County Water Authority is raising the 64-year-old San Vicente Dam in California to increase water supply reliability for southern California. The 220-foot-high concrete gravity dam will be raised by as much as 117 feet using roller-compacted concrete (RCC). Construction will begin by early 2009 and be completed by the end of 2012.

This dam raise, part of the Water Authority’s $3.4 billion Capital Improvement Program to increase water supply reliability for the region, will increase the usable capacity of San Vicente Reservoir by as much as 152,000 acre feet. The city of San Diego owns and operates San Vicente Dam and Reservoir for water supply purposes. After the dam raise, the Water Authority and the city of San Diego will jointly use the expanded reservoir.

This will be the largest raise of a concrete dam in the U.S. and the largest raise in the world using RCC. No concrete dam in the U.S. has been raised more than 77 feet, and that raise was performed using conventional concrete. To date, the largest raise of a dam in the U.S. using RCC is about 15 feet.

The work to raise San Vicente Dam comes on the heels of the Water Authority’s Olivenhain Dam project, which was completed in 2003 as the first phase to increase emergency storage for the region.1 Olivenhain Dam is the first major dam to be constructed in San Diego in more than 50 years, the first RCC dam built in California, and the tallest RCC dam in the U.S.2

Why raise San Vicente Dam?

The Water Authority’s mission is to provide a safe and reliable supply of water to its 24 member agencies in the San Diego region. The Water Authority imports water from hundred of miles away via aqueducts that cross several major fault zones. San Vicente Dam was constructed in 1943 as the first major storage reservoir for water imported into the region to meet the demands of a rising population, mostly due to military base expansions during World War II. Since then, the San Diego region’s dependence on imported water has grown to more than 90 percent.

The Water Authority is about halfway through implementation of its Emergency Storage Project (ESP) to mitigate a catastrophic interruption of imported water supplies, such as an earthquake that could sever supply aqueducts. The fourth and final phase of the 13-year-long ESP construction, which started in 1999, includes raising San Vicente Dam by 54 feet to add about 52,000 acre-feet of storage to the current capacity of 90,100 acre-feet.

To assist the region in meeting demand during the summer months, seasonal and carryover storage projects (storage to carry over water from periods of extended rainfall to periods of extended drought) are needed. As part of a master planning process completed in 2002, the Water Authority identified San Vicente Reservoir as a possible location to provide as much as 100,000 acre-feet of this increased carryover storage. Thus, San Vicente Dam may be raised further, subject to environmental review, to provide a total additional storage capacity of about 152,000 acre-feet to the reservoir. The dam would need to be raised a total of 117 feet above its current 220-foot height to accommodate this storage.

An agreement with the city of San Diego allows the Water Authority to raise San Vicente Dam for the ESP, and further if desired. The Water Authority is responsible for design, construction, and capital cost of the dam raise and will obtain rights to the additional reservoir capacity created. The city retains ownership and operation of the dam and reservoir. The city of San Diego also obtains increased flexibility in the use of its existing storage capacity by virtue of new conveyance facilities constructed for the ESP and the higher hydraulic head of the reservoir.

Existing features of the dam

San Vicente Dam is a conventional concrete gravity dam founded on bedrock that has a crest length of 963 feet. Concrete was poured in 5-foot-thick, 50-foot-wide blocks that extended from the upstream face to the downstream face of the dam. The spillway consists of a 275-foot-long uncontrolled ogee section near the center of the dam. Seepage control consists of a foundation grout curtain below the axis of the dam and vertical drains through the dam and foundation.

During design, the city of San Diego anticipated that the reservoir would need to be expanded in the future. Therefore, the city situated the dam axis to allow ample room for expansion on the downstream side and designed the length of the foundation grout curtain for a potential 120-foot raise of the spillway crest elevation.

The outlet works consist of three 36-inch-diameter cast iron conduits through the dam, connected on the upstream side to an outlet tower and on the downstream side to the city of San Diego’s San Vicente Pipelines 1 and 2. The outlet tower contains six saucer valves at various elevations to allow selective withdrawal of reservoir water.

Figure 1: The existing 220-foot-high San Vicente Dam will be raised to increase water storage in the reservoir. Total structural height of the dam could be as much as 337 feet. This raise will be accomplished using roller-compacted concrete.

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The dam raise will be accomplished by placing RCC against the downstream face and above the crest of the existing dam to create a composite concrete structure about 337 feet high, with a crest length of about 1,440 feet. (See Figure 1.) The raise involves foundation excavation on the downstream side of the dam, preparation of the surface of the dam to receive the new RCC, installation of additional seepage and drainage control elements, construction of a spillway to safely pass the inflow design flood, and construction of an outlet works system capable of simultaneous delivery of reservoir water to Water Authority and city supply systems.

Figure 2: In addition to raising San Vicente Dam, the San Diego County Water Authority plans to construct an interconnect pipeline between a new Water Authority pump station and the city of San Diego’s pipelines.

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Other elements associated with the raise are construction of two saddle dams in topographic low areas on the right side of the main dam, relocation of the city of San Diego’s marina complex used by recreational boaters, construction of paved access roads to the dam and new marina, and construction of an interconnect pipeline between a new Water Authority ESP pump station and the city’s pipelines. (See Figure 2)

The dam raise design and construction must be integrated with the city of San Diego’s existing conveyance system, as well as conveyance facilities being constructed by the Water Authority. The city’s conveyance system consists of mainly gravity flow pipelines, which must remain active during the dam raise. The raise must be accomplished while maintaining storage in the reservoir for use by the city in the event of a water supply emergency.

The Water Authority’s conveyance system will consist of a 21,000-horsepower pump station, a surge control facility on a hill that forms the right abutment of the dam, and an 11-mile-long pipeline to connect San Vicente Reservoir to the Water Authority’s aqueduct system. Water Authority conveyance facilities are under construction.

Getting permits for the construction project

The Water Authority’s planning and permitting for the raise of San Vicente Dam as part of the ESP was completed in 1998.

For the additional carryover storage, the planning phase consisted of engineering feasibility studies performed from 2004 to 2006 and ongoing environmental documentation/permitting, starting in 2005, for raising San Vicente Dam further. Permitting is expected to be complete in mid-2008. The Water Authority is the project proponent and local lead agency under the California Environmental Quality Act. The project requires the issuance of federal permits under Section 404 of the Clean Water Act. Therefore, the U.S. Army Corps of Engineers is the federal lead agency for compliance with the National Environmental Policy Act. The Water Authority is preparing an environmental impact report/environmental impact statement (EIR/EIS) to address the environmental issues associated with the carryover storage component. The EIR/EIS will assess potential effects associated with raising San Vicente Dam beyond the ESP volume and changes made to the ESP to accommodate carryover storage.

San Vicente Dam is under the jurisdiction of the State of California Division of Safety of Dams (DSOD) of the Department of Water Resources. DSOD will review plans and specifications for dam construction and must grant written approval before construction can proceed and before water can be impounded behind the raised dam. The Water Authority asked DSOD to review and approve several key design criteria during the planning phase of the project so that the final design process, once started, could proceed more efficiently.

Studies performed during the planning phase

The planning phase for the carryover storage project has included a significant amount of conceptual design engineering. This work was necessary to: assess the technical feasibility of raising San Vicente Dam by 117 feet, provide project information to support preparation of environmental documents, provide engineering data to support discussions between the Water Authority and the city of San Diego on the preferred configuration of facilities, and promote an efficient final design process.3 Key regulatory agency approvals were obtained from DSOD in the areas of geologic hazards, seismic criteria, and inflow design flood.

Condition of existing dam

GEI Consultants Inc. evaluated the condition of the existing dam in detail during the planning phase to identify issues that would influence the proposed raise. Fortunately, detailed records of the original dam construction (1941-1943) were available in city of San Diego and DSOD files. In general, the existing dam structure is in very good condition and well suited for the proposed raise.

Geotechnical exploration

GEI also performed a thorough geotechnical exploration program to obtain data for conceptual design of the dam raise and promote an efficient final design effort by examining key features of the foundation geology early in the planning process. Explorations were performed at the sites of the existing dam, proposed saddle dams, access roads, and potential quarries to provide aggregate for the RCC and conventional concrete.

The exploration program included borings through geologic contacts and within the dam, as well as borings and trenching through two shear zones in the foundation rock. Investigations included down-hole and surface geophysics, as well as installation of vibrating wire piezometers supplied by Geokon Inc. in seven borings located within the rock foundation. These piezometers were needed to measure subsurface uplift water pressures on the base of the dam.

The dam raise will require about 1.6 million tons of fine and coarse aggregate to produce the RCC and conventional concrete. GEI identified several potential aggregate sources for use, including off-site and on-site sources. Off-site sources would consist of materials mined from existing quarries in the area and trucked to the site. On-site sources would require development of a quarry near the dam and crushing operations to produce aggregate with suitable properties.

GEI also performed field testing of the dam’s foundation drainage system to obtain information on its effectiveness. This work included video inspections of the drains to look for signs of blockage. The Water Authority’s final designer will use the results of the testing program to evaluate the need for additional foundation drainage below the existing dam as part of the dam raise project.

Outlet works

The outlet works of San Vicente Dam consist of an outlet tower on the left side of the spillway that contains six intake ports. These ports are connected to three 36-inch-diameter cast iron conduits through the dam. The capacity of this outlet works is about 200 cubic feet per second (cfs). As part of the dam raise, it will be replaced by a new outlet works facility on the right side of the new spillway. (See Figure 3.) This new outlet works will have a capacity of about 630 cfs for water supply operations, as well as greater capacity for emergency evacuation of the reservoir to meet DSOD regulatory requirements.

Figure 3: The new outlet works to be installed at San Vicente Dam during the raise consist of a six-port tower affixed to the dam on the right side of the spillway and a low-level outlet to drain the reservoir at low water levels.

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One option would be to construct a 340-foot-high outlet tower on the upstream face of the dam to handle all the required flow capacity and allow the reservoir to be emptied in an emergency. However, the planning-phase concept GEI developed for this project places a shorter outlet tower, about 220 feet high, on the right abutment with its base at the approximate elevation of the reservoir level to be maintained during construction. This tower will connect to a conduit with a diameter of about 102 inches that passes through a tunnel under the existing dam. This tower will be much less expensive to build because it is shorter, eliminates the need for a cofferdam, and can be accessed by land via an access road on the right abutment. To drain the reservoir at low water levels, a 66-inch-diameter low-level outlet would be built through the dam.

Under contract to GEI, Flow Science Inc. performed a detailed water quality assessment of the expanded reservoir to establish the number, elevations, and capacities of ports to be placed on the new outlet tower for selective withdrawal purposes. This required considerable coordination among the Water Authority, the city of San Diego, and GEI to arrive at a solution that was cost-efficient and allowed flexibility in future reservoir operations.

Reservoir operations

The Water Authority, the city, and GEI are performing evaluations to define reservoir operations during construction, initial filling after construction, annual operations after initial filling, and operations during a water supply emergency event. This work will allow the Water Authority and the city of San Diego to integrate San Vicente reservoir operations into the management of its water supply reservoirs. The reservoir operations evaluation also provides data on reservoir fluctuations to tell recreational users of the reservoir when to expect the reservoir to be closed and re-opened for boating. First filling of the expanded reservoir will mainly be accomplished through imported water deliveries via the Water Authority’s aqueduct system. Complete filling is expected to take several years, starting in 2012.

Completing the final design

The Water Authority hired MWH Americas Inc. to prepare the final design of the San Vicente Dam Raise Project. MWH started its final design work in October 2006 and is expected to be completed in late 2009. Starting with the planning phase design concepts and key design criteria accepted by DSOD, the final design team is responsible for taking the project to construction. Work has started on key aspects, including RCC mix design and material investigations, supplemental field geotechnical/geological investigations, and establishing final design criteria for all project features.

One important first step for the final design team was a trip to Roosevelt Dam near Tempe, Ariz., which is the largest concrete dam raise in the U.S. The 77-foot dam raise was completed in 1995 by the U.S. Department of the Interior’s Bureau of Reclamation as part of the Salt River Project to add storage to meet hydrologic requirements.4 Reclamation conducted many key investigations for this project that will be applicable to the San Vicente Dam raise design, including extensive investigations into bonding of the interface between the existing dam and new concrete overlay. Reclamation demonstrated that foundation excavations could be made at the toe of the existing dam and that portions of the existing dam could be removed to accommodate the raised structure.

San Vicente Dam currently impounds a 90,100 acre-foot reservoir. The proposed dam raise will increase the reservoir capacity to 247,000 acre-feet, to provide emergency and carryover storage for the San Diego region.

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Another key aspect of the early design process is the RCC materials investigation. Constructability and construction economy demand critical evaluation of sources and performance of concrete aggregate, cement, and pozzolan. The dam raise will require a large amount of concrete. Therefore, optimizing the design to minimize the use of RCC materials can result in significant construction cost savings.

Two on-site sources of aggregate are available, but the materials are completely different. One borrow area consists of extremely hard granitic rock, similar to that used for Olivenhain Dam. The other borrow area is a conglomerate, consisting of hard rock cobbles in a sand matrix. Conglomerate cobbles were used to build the existing San Vicente Dam. Therefore, RCC using conglomerate as the aggregate should more closely match the thermal properties of the existing concrete. Initial constructability reviews by MWH indicate that the conglomerate material will be much more economical to mine and crush than the granitic rocks.

One of the key lessons the Water Authority learned on the Olivenhain Dam project was that early construction packages could shorten the overall construction period and reduce the overall cost of construction. To that end, MWH has proposed three packages of construction for the San Vicente Dam raise project, including:

– Package 1: Test Quarry and Phase II RCC Mix Design Program. This involves development of an on-site test quarry to gather information and crush rock for use in a final RCC mix design laboratory program.

– Package 2: Foundation Excavation. This involves excavation of the foundation footprint for the raised dam, the stilling basin, a downstream control facility, and the base for the new outlet tower. This package also includes construction of a marina access road and demolition of the existing marina.

The Water Authority has selected a seven-member independent board of senior consultants to review key aspects of design. The Water Authority plans to engage a construction management firm to assist in the design and constructability review process, and to provide services during construction. The Water Authority plans to have this firm retained before the end of 2007 so that it can participate in a value engineering session to be conducted during the preliminary design phase, which is currently under way.

Considering a hydro facility

The topography surrounding San Vicente Reservoir makes development of pumped-storage hydroelectric power generation potentially viable. In fact, the city of San Diego worked with another entity that obtained a preliminary permit for this type of project from the Federal Energy Regulatory Commission (FERC) in 1993. That permit expired, and an extension is not being pursued.

On March 1, 2007, the Water Authority was granted a preliminary permit from FERC to secure a priority position for development of a pumped-storage project at the San Vicente site, known as the San Vicente Pumped-Storage Water Power Project. San Vicente Reservoir would form the lower reservoir, and several locations would be considered for the upper reservoir.

After the preliminary design for the dam raise is complete in 2007, the Water Authority plans to initiate a marketing study to determine the economic feasibility of developing this project. The initial study may look into possible generation up to 500 MW; the study also should recommend an optimum range for the power generation development. Should such studies indicate a pumped-storage project at the San Vicente Reservoir site is economically viable, the Water Authority would pursue the project as a separate capital improvement project, and separate engineering studies and environmental investigations would be required.

San Vicente could become the Water Authority’s second pumped-storage project. The Water Authority is already constructing a 40-MW pumped-storage plant, known as the Olivenhain-Hodges Pumped Storage Project, about 15 miles northwest of San Vicente Reservoir.1 This project should be complete and operational by November 2008. n

Tom Keller, vice president of GEI Consultants Inc., was GEI’s project manager for planning-level engineering evaluations and designs for raising San Vicente Dam. Jeff Shoaf is senior engineer and Jim Zhou is design manager with the San Diego County Water Authority. Shoaf and Zhou were responsible for the Water Authority’s overall project management and engineering design management, respectively. Jesus Meda, senior civil engineer with the City of San Diego Water Department, is responsible for city review and coordination of the dam raise. Mike Rogers, P.E., is vice president with MWH and project manager for the final design effort.